The compression ratio of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine. In conventional engines, the compression ratio is fixed and thus the engine efficiency cannot be optimized during operating conditions to improve fuel economy and engine power performance.
Various technologies have been developed to enable the compression ratio of an engine to be varied with engine operating conditions. One example approach is shown by Yoshida et al. in U.S. Pat. No. 7,258,099. Therein, cam timing adjustments are used to vary the effective compression ratio. For example, a late intake valve closing is used to reduce the effective compression ratio. Still other approaches, such as shown by Kamada et al. in US20130055990, rely on a piston displacement changing mechanism that moves the pistons closer to or further from the cylinder head, thereby changing the size of the combustion chambers.
However the inventors herein have recognized potential issues with such approaches. As one example, there may be constraints and trade-offs associated with the cam timing adjustments of Yoshida, such as reduced volumetric efficiency, torque, and power when low compression ratio is desired. Another issue is that frequent changes in operator pedal demand may cause the engine load to move back and forth, leading to frequent switching between the compression ratios. Excessive compression ratio switches can degrade fuel economy due to losses incurred during transitions. The issue may be exacerbated in a hybrid vehicle where the engine encounters multiple engine pull-ups and pull-downs (such as during frequent start/stop events). The fuel losses during pull-ups and pull-downs may be proportional to pumping and friction work of the engine. Yet another issue associated with the frequent engine pull-ups and pull-downs is that a bobble can occur as the engine passes through the low speed range (e.g., between 300-500 rpm). This rapid and repeated temporary speed fluctuation (relative to an average speed change passing through the window) is due to torque pulsations from engine compression-expansion cycles that excite the vehicle in that speed range, especially in the common hybrid powertrain designs, such as the power-split, that have a direct mechanical connection between the engine and the wheels.
The inventors herein have recognized that a variable compression ratio (VCR) engine, such as one configured with a mechanism that mechanically alters a piston position with a combustion chamber, can be leveraged in a hybrid vehicle system to reduce the compression ratio during engine pull-up and pull-down events without being hindered by associated constraints and trade-offs. At the same time, battery power can be leveraged to reduce the frequency of compression ratio switching. In one example, fuel economy may be improved by a method for a hybrid vehicle system comprising shifting between propelling the vehicle via motor torque and engine torque responsive to driver demand; and during the shifting, when engine speed is at or below a threshold speed, transitioning the engine to a lower compression ratio via mechanical adjustments. In addition, the controller may select between maintaining a given compression ratio or transitioning to the other compression ratio based at least on a system battery state of charge. As a result, frequent compression ratio switching can be reduced.
As an example, a hybrid vehicle system may be configured with a battery powered electric motor for propelling vehicle wheels via motor torque, as well as a VCR engine for propelling vehicle wheels via engine torque. The VCR engine may include a VCR mechanism for mechanically altering a compression ratio of the engine, such as by altering a position of a piston within a cylinder, or altering a cylinder head volume, as non-limiting examples. During conditions when the engine is being pulled-up (such as during a transition from electric mode to engine mode), as well as when the engine is being pulled down (such as during a transition from engine mode to electric mode), the engine may be operated with a lower compression ratio. In particular, the lower compression ratio setting may be selected and held during the engine pull-up until the engine speed exceeds the bobble region (e.g., between 300-500 rpm). Likewise, during the engine pull-down, the engine may be transitioned to the lower compression ratio setting just before the engine enters the bobble region. Once outside the bobble region, a vehicle controller may select a compression ratio that provides the highest fuel economy for a given torque demand. This may include, for example, in response to a change in torque demand, providing engine torque while maintaining a current compression ratio setting and while additionally applying an amount of motor torque to meet the driver torque demand.
In this way, fuel economy losses in a vehicle system can be reduced. One of the technical effects of using VCR technology in a hybrid vehicle is that the compression ratio can be reduced during the frequent engine pull-ups and pull-downs with fewer constraints and trade-offs. The lower compression ratio during the start/stop event results in lower cylinder pressure, which decreases pumping work (the work to compress and expand the cylinder air), piston ring friction and piston side loads, thereby improving fuel economy. In addition, heat transfer losses and blow-by during engine pull-up/pull-down is reduced, lowering the negative in-cylinder mean effective pressure (IMEP) in the compression/expansion loop. This enables friction and pumping losses to be reduced, and improves noise, vibration, and harshness (NVH). Furthermore, by applying the lower compression ratio in the lower engine speed region, torque pulsations (such as those incurred when the engine passes through the bobble region) are reduced. The technical effect of using battery power to meet driver demand while maintaining an engine compression ratio during selected engine operating conditions is that compression ratio switching can be reduced. In addition, engine operation in a more fuel efficient compression ratio can be extended despite changes in driver or wheel torque demand.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.